SWR meters, feedline length, antenna tuners, forward and reflected power, matches, mismatches, and conjugate matches etc are sometimes poorly understood. The ARRL website has many articles on these subjects, found under the “Transmission Lines” subject heading.
Some articles are simple, some are complex. The math behind all this would fill a couple of blackboards – ask Dr. Maxwell.
What is SWR? It is the ratio of the forward power (sent to the antenna) to the reflected power (bounced back from the end of the transmission line). So if 100 watts is sent out and 50 watts bounces back, the SWR ratio would be 1:2. More about the reflected power below…
What does an antenna tuner do? Modern transmitters are designed to work into a 50 ohm load. Their output tuning will accommodate a little more and a little less than 50 ohms, and will automatically reduce the output power as the load varies significantly from 50 ohms.
The feedline to the antenna often does not present a 50 ohm load to the transmitter. So we must put a matching network between the transmitter and the feedline. This is usually called an antenna tuner (ATU).
The ATU is misnamed; it does not tune an antenna; it provides what is called a conjugate match between the transmitter and the feedline so that the transmitter “sees” a resistance it likes when its output hits the ATU.
What’s a conjugate match? If you love math and really must know, KL7AJ explains it here. Simply put, a conjugate match is achieved when the reactive portion of the load (at the point where the transmission line attaches to the ATU) is matched by applying an equal but opposite reactance to that point. This cancels the reactive component of the load so that only the resistive portion is left and Ohm’s law can now be applied.
Once a match is achieved (indicated by about a 1:1 match on the SWR meter placed between the transmitter and the ATU, the transmitter can deliver 100% of its power and it will be happy. But note, it is delivering its power into the ATU, not the transmission line.
Assuming the ATU is properly adjusted (“tuned”) so that the power put into it is passed to the transmission line and not just heating the ATU’s internal components, the transmitter’s output power will now pass through the tuner and be dumped into the transmission line.
This power races along the transmission line until it hits the junction of the transmission line and the antenna. If the resistance of the antenna and the resistance of the transmission line are not equal at this frequency (they often are not), some of the power will bounce back down the transmission line to the ATU. This is reflected power. It will be out of phase with the forward power.
What happens to the reflected power? It bounces off the ATU and is added to the forward power going up the transmission line and again is sent to the antenna. If you want the extensive math behind this, ask Dr Maxwell.
The point is that if there is approximately a 1:1 SWR between the transmitter and the ATU, and if the ATU is properly tuned, then the transmitter’s power (both forward and reflected) will be radiated by the antenna.
The design of the antenna then determines the take-off angle and directivity of the radiated power.
Additional comments:
An external device, like a Field Strength Meter is easy to build and can help confirm that you are in fact radiating power, not just using it to heat the tuner. If the tuner feels warm, it’s not a good match.
If you experience “RF in the shack,” the feedline is probably radiating, indicating a mismatch problem or that the two sides of the ladder line or antenna are not equal. Read W8JI’s extensive article that addresses this problem.
Devices like RF ammeters are good to use but they are hard to come by and are expensive.
Ladder Line exhibits very little loss to both the forward and reflected power; much less than coax, so it can be a very good choice, where a mismatch at the junction of the transmission line and the antenna is expected (as in a multiband doublet, for example).
We said that the forward and reflected power can be out of phase with each other but along the transmission line they will tend to ADD and to SUBTRACT from each other at different points. Where they add produces a high RF voltage point.
It is not good practice to attach the transmission line to the ATU at the point on the transmission line where there is a high voltage. It is best to select a point where the two phases have subtracted, producing a low voltage point.
KL7AJ’s article below gives a much better (and longer) explanation of all this.
Another page in the antenna section of this website might help select antenna and transmission line lengths, so that you can have a low voltage at the ATU attachment point on all bands.
Here’s a couple of articles that might help:
KL7AJ’s “SWR Meters Make You Stupid” is a light-hearted but thorough explanation of standing waves, phases, forward and reflected powers and how the antenna, transmission line, tuner and transmitter all work together to radiate your power.
W1DX’s “My Feedline Tunes my Antenna” article is slightly more technical but it’s very good. And no, the feedline does not tune the antenna.